Bio chapter 21 b

Chapter 21: The Evolution of Populations

Testing Population Evolution

  • To test if a population is evolving, we utilize the Hardy-Weinberg Equation.

  • In a population that is not evolving, allele and genotype frequencies remain constant from generation to generation.

  • This constancy is referred to as the Hardy-Weinberg equilibrium.

  • The Hardy-Weinberg equation requires knowledge of the frequency of alleles in a population.

    • Allele Frequencies:

    • Let p represent the frequency of the CR allele.

    • Let q represent the frequency of the CW allele.

    • Example values:

      • p = 0.8 (frequency of CR allele)

      • q = 0.2 (frequency of CW allele)

Gamete Production

  • Gametes produced:

    • Each egg has an 80% chance of carrying the CR allele and a 20% chance of carrying the CW allele.

    • Each sperm has the same distribution:

    • 80% chance of carrying the CR allele.

    • 20% chance of carrying the CW allele.

Frequencies of Genotypes

  • To evaluate if a population is evolving, we need to determine the genotype frequencies:

    • Homozygous Dominant (CRCR):

    • p^2 = (0.8)^2 = 0.64

    • Heterozygous (CRCW):

    • 2pq = 2(0.8)(0.2) = 0.32

    • Homozygous Recessive (CWCW):

    • q^2 = (0.2)^2 = 0.04

Example Population of Cats

  • A population consists of black and white cats, where the black allele (B) completely dominates the white allele (b).

  • Given:

    • Total cats = 1,000

    • Black cats = 840

    • White cats = 160

Allele Frequency Calculations

  • Frequency of the Dominant Allele (B): p = 0.6

  • Frequency of the Recessive Allele (b): q = 0.4

  • Genotype Frequencies: To calculate the frequency of individuals with each genotype:

    • Homozygous Dominant (BB): p^2 = 0.36

    • Heterozygous (Bb): 2pq = 0.48

    • Homozygous Recessive (bb): q^2 = 0.16

Calculation Breakdown

  1. Determine the frequency of individuals with the bb genotype:

    • There are 1,000 cats, with 160 being bb. Thus, q^2 = 160/1000 = 0.16

  2. Calculate q:

    • q = ext{sqrt}(0.16) = 0.4

  3. Solve for p:

    • p + q = 1
      ightarrow p + 0.4 = 1
      ightarrow p = 0.6

  4. Find p^2:

    • 0.6 imes 0.6 = 0.36

  5. Calculate 2pq:

    • 2 imes 0.6 imes 0.4 = 0.48

Hardy-Weinberg Equilibrium Evaluation

  • In the next generation, a population of 800 cats has:

    • Black cats: 672

    • White cats: 128

  • To check for Hardy-Weinberg equilibrium:

    1. Calculate q^2:

    • Number of bb = 128, so q^2 = 128/800 = 0.16

    1. Calculate q again: ext{sqrt}(0.16) = 0.4

    2. Compare previous q: They are equal, indicating that the population is in Hardy-Weinberg equilibrium.

Conditions for Hardy-Weinberg Equilibrium (KNOW)

  • Necessary Conditions:

    • No mutations.

    • Random mating.

    • No natural selection.

    • Extremely large population size.

    • No gene flow.

  • These conditions are rare in natural populations.

Mechanisms of Change in Allele Frequencies

  • Allele frequencies can change in populations via three mechanisms:

    • Genetic Drift

    • Gene Flow

    • Natural Selection

Genetic Drift

  • Definition: A chance event that can cause allele frequencies to fluctuate unpredictably from one generation to the next.

Founder Effect

  • Definition: Occurs when a few individuals become isolated from a larger population, creating a new population whose gene pool may differ from the source population.

Bottleneck Effect

  • Definition: A sudden environmental change causes drastic reduction in population size, resulting in certain alleles being overrepresented or underrepresented randomly.

  • Example: Northern elephant seals hunted to near extinction lost genetic diversity due to small remaining population.

Effects of Genetic Drift/ 4 kay points to genetic drift:

  1. Significant in small populations due to low allelic diversity.

  2. Can lead to random changes in allele frequencies.

  3. May result in the loss of genetic variation.

  4. Can fix harmful alleles permanently.

Gene Flow

  • Definition: The transfer of alleles into or out of a population due to movement of fertile individuals or their gametes.

Natural Selection

  • Definition: A process that consistently increases the frequencies of alleles providing reproductive advantage, leading to adaptive evolution.

  • Key Concept: Survival of the fittest links to relative fitness, defined as the contribution an individual makes to the gene pool of the next generation compared to others.

  • Acts more directly on phenotype than genotype.

Types of Natural Selection

  1. Directional Selection: Favors individuals at one extreme of a phenotype range.

  2. Disruptive Selection: Favors individuals at both extremes over intermediates.

  3. Stabilizing Selection: Favors intermediate variants over extremes.

Natral selection is not random:

Natural selection consistently increases the frequencies of alleles that provide reproductive advantage, thereby leading to adaptive evolution.

‘Survival of the fittest’

•Certain traits can lead to greater relative fitness: the contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals.

Natural selection acts more directly on phenotype than genotype. This means that even if an advantageous allele is present, it must be expressed in a way that enhances survival and reproduction to have an impact on evolution.

Other Selection Mechanisms

  • Balancing Selection: Maintains variation in alleles at specific loci, preserving multiple phenotypic forms.

    • Heterozygote Advantage: Heterozygous individuals exhibit greater fitness than homozygous individuals.

    • Frequency-Dependent Selection: Fitness depends on the frequency of phenotypes (e.g., predation impacts on common vs. rare morphs).

Sexual Selection

  • Definition: Individuals with inherited traits are more likely to attract mates.

  • Sexual Dimorphism: Differences in traits (size, color, behavior) between sexes due to sexual selection.

Two types of sexual selection:

  1. Intrasexual Selection: Competition among individuals of the same sex for mates, often involving displays of strength or other traits that signal fitness. Intra=within ; roles typically involve males competing for females, which can manifest in various forms such as aggression or elaborate displays.

  2. Intersexual Selection: Preferences by one sex for specific qualities in mates, leading to the evolution of pronounced traits in the other sex. Mate choice, individuled of one sex and choosy in choosing there mates from the other sex, usualy females choose males.

Natral selection cannot fashion perfict organisms because:

  • Limitations:

    1. Selection only acts on existing variations.

    2. Historical constraints limit evolutionary pathways.

    3. Adaptations may be compromises.

    4. Interaction between chance, natural selection, and environmental factors.